This invention relates to processes for modifying the surface of an article. More particularly, this invention is directed to peening processes by which mechanical properties and surface finish characteristics of a component can be improved.
Shot peening is a process by which the surface and immediate underlying substrate regions of a component can be modified to exhibit improved properties, including improved resistance to fatigue and foreign object damage by inducing compressive residual stresses. Certain components of turbomachinery, including airfoil components such as gas turbine blades, steam turbine blades, and gas turbine engine blades formed of steel, titanium-based alloys and superalloys, may require complete shot peening of their airfoil surfaces at relatively high intensities, for example, an Almen intensity of 10N on the Almen N strip scale (about 3A on the Almen A strip scale) or higher, to obtain the desired surface properties (all peening intensities referred to herein are quantified on either the Almen A or N strip scale). However, shot peening at high intensities tends to cause significant surface roughening of an airfoil surface, for example, about 90 microinches (about 2.3 micrometers) Ra and greater, which can be detrimental to blade aerodynamics and the overall performance of the turbine. Increased surface roughness also promotes the adhesion of airborne contaminants, corrodents, and erodents whose deposits can promote crevice pitting, stress corrosion cracking and fatigue loss.
In order to reduce roughness following peening, compressor blades often undergo a polishing process, such as prolonged tumbling, hydro-honing, drag finishing, chemical etching, or other methods to reduce the surface finish to more acceptable levels, for example, 35 microinches (about 0.9 micrometers) Ra. However, the resulting surface finish is often higher than the original pre-peened airfoil surface finish. In addition to increasing the production costs and cycle time, post shot-peen polishing processes can also negate the benefits obtained from shot peening by removing the compressive residual stress layers, and in so doing can also cause dimensional distortion.